Document WO 2009/011808 A1 describes a method for determining an activity for attaching a protein within a drop.
The publication “Single-cell analysis and sorting using droplet-based microfluidics”, of Mazutis et al. published online, on Apr. 4, 2013 in the journal Nature Protocols illustrates this principle.
A mouse hybridoma is encapsulated in a drop with a bead covered with anti-mouse antibody. The hybridoma secretes antibodies. A secondary antibody coupled with a fluorophore gives the possibility of revealing the presence of the secreted antibody. The distribution of the secondary antibody is, in the absence of a secreted antibody, homogeneous in the drop, but is relocalized on the bead in the presence of antibodies.
The method is therefore very selective for determining the activity of a particular cell.
On the other hand, such a method has diverse drawbacks. The method for compartmentalizing the cells and the beads is random. The number of beads in the drops may be estimated by a Poisson distribution law. Also, the number of cells within the drops may be estimated by an independent Poisson distribution law. The initial concentrations of beads and drops are adjusted for having on average one cell and one bead per drop. Only a portion of the drops therefore is of interest for the achieved analysis.
Moreover, the presence of a single bead of a significant size per drop is not favorable to the resolution of the method. Indeed, the secondary antibodies are distributed over the whole surface of the bead. The dynamic range of the method is therefore limited by the external surface area available per bead.